Table Of ContentRocks and Minerals
in Thin Section
Second Edition
W.S. MacKenzie
Emeritus Professor of Petrology,
University of Manchester, England
A.E. Adams
Senior Lecturer in Geology (retired),
University of Manchester, England
K.H. Brodie
Senior Lecturer in Geology,
University of Manchester, England
Revised 2nd edition
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Contents
Preface, v Basalt, 74
Acknowledgements, v Olivine basalt, 76
About the authors, vi Basalt, 78
Second edition, vii Dolerite, 80
Introduction, viii Olivine gabbro, 82
Gabbro, 84
Andesite, 86
Part 1 Optical mineralogy, 1
Diorite, 88
The polarizing microscope, 1
Granodiorite, 90
Description of minerals, 2
Rhyolite, 92
Shape and habit of crystals, 2
Microgranite, 94
Colour and pleochroism, 6
Granite, 96
Cleavage, 8
Alkali granite, 98
Relief, 10
Phonolite, 100
Birefringence, 14
Nepheline syenite, 102
Extinction angles, 18
Lamprophyre, 104
Twinning and zoning, 20
Ignimbrite, 106
Undulose extinction and sub-grain
structure, 22
Alteration, 24
Part 4 Sedimentary rocks, 109
Grain size, 26
Terrigenous clastic rocks, 109
Carbonate rocks, 112
Quartz arenite, 116
Part 2 Minerals, 27
Feldspars in sedimentary rocks, 118
Olivine, 28
Sub-arkose, 120
Orthopyroxene, 30
Arkose, 122
Clinopyroxene, 32
Sub-litharenite, 124
Two-pyroxene intergrowth, 34
Litharenite, 126
Amphibole, 36
Wacke/Greywacke, 128
Biotite, 38
Micaceous sandstone, 130
Muscovite, 40
Calcareous sandstone, 132
Chlorite, 42
Glauconitic sandstone, 134
Quartz, 44
Mudstone, 136
Feldspars, 46
Ooid grainstone, 138
Sanidine, 48
Ooid packstone, 138
Microcline, 50
Bioclast packstone, 140
Plagioclase, 52
Bioclast wackestone, 140
Nepheline, 56
Intraclast grainstone, 142
Calcite, 58
Peloid grainstone, 142
Garnet, 60
Carbonate mudstone, 144
Opaque minerals, 62
Dolomite, 146
Radiolarian chert, 148
Part 3 Igneous rocks, 65 Replacement chert, 150
Peridotite, 72 Evaporite, 152
iii
Ooidal ironstone, 154 Andalusite-cordierite hornfels, 198
Banded ironstone, 156 Garnet-cordierite hornfels
Volcaniclastic rocks, 158 (Granofels), 200
Porosity in sedimentary rocks, 160 Metamorphosed carbonate rocks, 202
Forsterite and diopside marble, 204
Part 5 Metamorphic rocks, 163 Metamorphosed basic rocks, 206
Textures of metamorphic rocks, 168 Garnet epidote amphibolite, 208
Schistosity and schists, 170 Amphibolite, 210
Gneissic foliation and gneisses, 172 Two pyroxene granofels, 212
Mylonite, 174 Glaucophane-lawsonite-schist
Cataclasite, 176 (Blueschist), 214
Crenulations, 178 Glaucophane-epidote (Blue) schist, 216
Lineations, 180 Eclogite, 218
Reaction textures, 182 Ultramafi c rocks, 220
Metamorphosed mudstones (pelites), 186
Chloritoid schist, 188 Appendices, 223
Garnet mica schist, 190 Appendix I, 224
Staurolite schist, 192 Appendix II, 226
Kyanite gneiss, 194
Garnet-sillimanite gneiss, 196 Index, 229
iv
Preface
This atlas has been prepared for students of earth science, geology, mineralogy and
physical geography who require a text for practical classes on rocks and minerals
under the microscope. While the book’s prime purpose is as an introduction to the
subject for college and university students as an essential part of their course, we
hope that amateur geologists and mineralogists will also fi nd it useful and attractive.
We have tried to make the text and pictures self-contained such that an individ-
ual who has access to a polarizing microscope and a collection of thin sections of
rocks can begin recognizing minerals and naming rocks without supervision. Our
aim has been to provide a manual for use in practical classes by showing illustra-
tions of some of the diagnostic properties of minerals and introducing the most
common rock-forming minerals. We then illustrate a representative selection of
igneous, sedimentary and metamorphic rocks.
We have deliberately limited the scope of the introduction to optical mineralogy
and have assumed little knowledge of crystallography or physical optics. We would
hope, however, that the coverage will encourage students to study the elements of
crystal symmetry and thus be in a better position to understand crystal optics. This
would assist the student to progress to the use of optical techniques not covered
here, such as the use of convergent light.
Most of the photographs of rocks in this book have been taken at low magnifi -
cation to illustrate representative views of the constituent minerals and their inter-
relationships. The photographs were taken either in plane-polarized light or under
crossed polars: in many cases the same fi eld of view is shown under both condi-
tions. Some of the photographs reproduced here are from thin sections which have
been used previously for other publications. However, all the photographs here
were made especially for this publication as 6 × 9 cm transparencies. We have had
the advantage of having access to a Zeiss Ultraphot microscope for this purpose.
Acknowledgements
Most of the thin sections illustrated are from the teaching collections of Manchester
University Geology Department, and we are indebted to our colleagues who have
collected these specimens over many years. We are especially grateful to those who
have supplied us with additional material, particularly Giles Droop, Alistair Gray
and John Wadsworth.
Colin Donaldson kindly agreed to read the fi rst two sections and made useful
comments on the text. The authors alone, however, are responsible for the choice
of rock types and for their descriptions.
We also record our grateful thanks to Carolyn Holloway for her typing of the
text and for her patience during all our changes of mind.
v
About the authors
Professor William MacKenzie’s main research area was the application of high
pressure-temperature experiments to petrology, with a particular interest in proper-
ties of high temperature alkali feldspars. He established a world-renowned exper-
imental petrology laboratory at Manchester in 1956. His interest and expertise in
microscopy and photography combined with his enthusiasm for teaching, led to the
publication of a series of Atlases of rock forming minerals. Many of his original
plates have been retained in this new edition.
Dr Anthony Adams was a popular lecturer in sedimentology at the University of
Manchester for much of his career until his retirement. His research was focused on
carbonate sedimentology, with a special interest in carbonate rocks of south Wales
and the western Mediterranean.
Dr Katharine Brodie has taught petrology for the last 40 years, fi rst at Imperial
College, London and subsequently at the University of Manchester. Her main
research interest has been the interactions between metamorphism and deforma-
tion, linking laboratory experimental work with natural examples, on scales rang-
ing from microscopic to larger scale tectonic processes.
vi
Second edition
In producing a second edition, many of the original micrographs have been retained
but the classifi cation of igneous and of metamorphic rocks has been updated to
refl ect current usage. Mudstones and porosity has been added to the sedimentary
section and the metamorphic section has been expanded. The new photographs
have been taken using a Nikon XX microscope.
Giles Droop, Alison Pawley and Brian O’Driscoll are thanked for providing
additional samples and comments on the revised version.
vii
Introduction
To gain an introduction to the identifi cation of minerals and rocks under the polar-
izing microscope, the student has fi rst to acquire some knowledge of the com-
pound microscope in its simplest form. This may sound like a contradiction in
terms but a magnifying lens is correctly described as a simple microscope, whereas
a compound microscope has at least two lenses, one producing a real image of
the object (the objective lens) and the other magnifying this image (the eyepiece).
Magnifi cations greater than 20 times are normally obtained using a compound
microscope. It is assumed that the operations of focusing the microscope, adjusting
the illumination and ensuring that the centring of the stage with respect to the optic
axis of the microscope can all be accomplished. It is also assumed that the student
has access to a collection of thin sections of rocks ground to the standard thickness
of one thousandth of an inch or 0.03 mm.
First we hope to help the student to describe minerals. After only a few hours
study the beginner will guess the identity of some minerals as he or she becomes
familiar with the appearance of the commonest minerals under the microscope by
observation of their properties. This can work satisfactorily as long as the student
can describe the optical properties correctly–if one or more of the properties do not
correspond to the mineral suggested then the identifi cation is incorrect and he or
she must think again.
Rocks are composed of aggregates of minerals. After determining the minerals,
the identifi cation of a rock depends on the relative abundance of the minerals and
on the textural relationships between them. No attempt has been made to introduce
the student to petrogenesis, i.e., the study of the origins of rocks. Our aim is to intro-
duce the subject of petrography or the description of rocks, since it is extremely
important to distinguish observations from hypotheses, and the observations must
come fi rst. However, some simple assumptions about the origins of rocks must be
made before they can be classifi ed, but these are unlikely to be controversial. A
short account of the nomenclature of the rocks is given at the beginning of each
section. A brief introduction to textural observations is provided as more detailed
observations and interpretations are beyond the scope of this book.
We have not given a complete petrographic description of any rock because
this can only be written after examination of an actual thin section of rock viewed
at different magnifi cations and covering an area representative of the whole rock.
viii
Eye pieces
Analyser lever
Rotating
nose-piece with Rotating stage
three objectives
Coarse and fine
focus
Substage
Iris diaphragm
Light source
A student-model petrographic microscope.
ix
